On the offchance anybody was planning on picking this up at some point it only seems to be 400MS at the moment if you've a Gold account (get it here)
No idea why this is reduced but it seems to be another Microsoft stealth deduction, possibly part of a sale they're about to reveal maybe .... or they could just be feeling the pinch from having 3 weeks of crap for DotW. Who knows?
Anyway I invested and have to say its a decent little game, well worth the price. If you like twin-stick shooters with a bit of a steampunk vibe then this'll be bang up your street